Better late than never I suppose. But, I will keep it simple and sweet. There are two things I remember my Mom saying to me growing up:
"You can't change the other person. All you can do is change yourself."
This was particularly in reference to my father. I couldn't change him, but I can make sure I don't have to live with the guilt of being part of the problem.
"Don't end your 'sorry' with a 'but'."
That takes repentance and turns it into an excuse. Say I'm sorry...Period. No explanations, no reasoning. Just admit that you were wrong. Leave it up to the other person to forgive and understand and extend grace.